1. Field of the Invention
The present invention relates to an optical information reproducing device for reading informations recorded on an opto-magnetic recording medium.
2. Related Art Statement
Such an optical information reproducing device is disclosed in, for example, Japanese Patent Application Opened No. 227,230/89, and is shown as a conventional example in FIG. 9 of the present invention. In the first conventional device, the diverging beam from a laser diode 1 is incident on a beam splitter 3, the beam reflected by the beam splitter 3 is converged by an objective lens 4 and illuminated on a disk 2. The return beam reflected by the disk 2 is incident on the beam splitter 3 through the objective lens 4 to separate the light transmitted through the beam splitter from an outward path of the light, the return light transmitted through the beam splitter 3 is incident on a plane-parallel plate 5, and the return light (first light beam) reflected on a surface 5a of the plate 5 and the return light (second light beam) refracted and transmitted through the surface 5a and reflected on an inner surface 5b of the plate 5 and further refracted and transmitted through the surface 5a are received on a photodetector 6.
The plane-parallel plate 5 is so constructed that light quantity of the first light beam is equal to that of the second light beam. As shown in FIG. 10, the photodetector 6 comprises a first light receiving element 7 for receiving the first light beam and a second light receiving element 8 for receiving the second light beam and is disposed at substantially a center between a focus F1 of the first light beam and a focus F2 of the second light beam in such a manner that when the objective lens 4 is in a focusing condition to the disk 2, the first light receiving element 7 is disposed in front of a focal plane of the first light beam and the first light receiving element 8 is disposed behind a focal plane of the second light beam. In this case, a light receiving surface of each of the respective light receiving elements is formed in accordance with a sectional area of the corresponding light flux in such a manner that light outputs of the first and second light receiving elements 7, 8 are equal to each other. In this way, the focusing control is performed on the basis of the difference between the light outputs of the first and the second light receiving elements in the first conventional example.
Also, a second conventional optical information reproducing device is disclosed in, for example, Japanese Patent Application Opened No. 185,750/90, and is shown as a conventional example in FIG. 11 of the present invention. In this second conventional example, the diverging beam from a laser diode 22 is made a collimated beam by passing through a collimator lens 23, and then is incident on a first surface 26a of a square column prism 26 through a half-wave plate 24. The beam refracted and transmitted through the first surface 26a and emitted from a second surface 26b is converged by an objective lens 25 and then illuminated on an opto-magnetic disk 21. The return light reflected from the opto-magnetic disk 21 is incident on the second surface 26b through the objective lens 25 and reflected on the first surface 26a in the prism 26 and then led to a third surface 26c therein. The return beam refracted and transmitted through the third surface 26c of the prism 26 is received on a first photodetector 29 having light receiving region divided by four through a condenser lens 27 and a circular column lens 28, while the return light reflected on the third surface 26c and refracted and transmitted through a fourth surface 26d of the prism 26 is received on a second photodetector 30 having light receiving region divided by two.
In this case, the first surface 26a of the prism 26 is formed as a half-mirror surface having a function by which the beam having an ellipsoidal cross-section emanated from the laser diode 22 is shaped to a beam having a circular cross-section. The third surface 26c of the prism 26 is formed as a surface of polarization beam splitter by a dielectric multilayer coating having about 100% of p polarization transmittivity and about 100% of s polarization reflectivity. The fourth surface 26d of the prism 26 is formed as a polarization beam splitter surface having about 100% of s polarization transmittivity. As described above, in the second conventional example, the information reproduced signal is detected on the basis of the difference between the sum output of the light receiving regions divided by four of the first photodetector 29 and the sum output of the light receiving regions divided by two of the second photodetector 30, the focusing error signal is detected on the basis of the difference of the sum output of the opposite angle regions of the light receiving region divided by four of the first photodetector 29, and the tracking error signal is detected on the basis of the output difference of the light receiving regions divided by two of the second photodetector 30.
As shown in FIG. 12a, in the construction shown in the first conventional example, the circular spot of a beam is formed on the first light receiving element 7 and the ellipsoidal spot of the beam is formed on the second light receiving element 8 because of having astigmatism caused by passing the second beam through the plane-parallel plate 5, so that respective spots include push-pull signal components. As shown in FIG. 12a, therefore, if at least one of the first and second light receiving elements 7, 8 (both elements 7, 8 in FIG. 10a) is constructed by the light receiving region divided by two in the direction of the push-pull signal, the tracking error signal can be detected by the push-pull system on the basis of the output difference of the light receiving region divided by two.
In this first conventional example, however, the device is so constructed that the minor axis direction of the spot having the ellipsoidal cross-section of the second light beam having astigmatism is equal to the push-pull signal direction, so that if the optical component such as laser diode 1 or the other is inclined by the change or so of the outer circumstance and the optical axis is shifted in the direction of the push-pull signal, as shown in FIG. 10b, the push-pull signal is liable to subject to an offset in the second light receiving element, and thus the tracking error signal can not be detected precisely. Moreover, the light receiving surface of the first and second light receiving element is formed in accordance with the cross-section of the corresponding light flux, so that respective light spot is liable to shift from the corresponding light receiving element due to the shift of the optical axis, and thus the modulation ratio of the push-pull signal is decreased and when the information signal is detected with sum of light quantity, C/N of the information signal becomes deteriorated too.
In the construction shown in the second conventional example, moreover, the first surface 26a of the prism 26 is made as a half-mirror surface, so that in case of reflecting the return light on the first surface 26a, a half of the light including the information signal component has been transmitted and thus the C/N of the information signal becomes deteriorated by 6 dB.